0.2.0

A saved answer now records which layout it was written in and which specsolve
wrote it, so a later release can name an answer it no longer reads. The Python
API reference is now rendered from the docstrings.

This release breaks one thing:

• An answer or an archive that 0.1.0 or earlier wrote is refused with a
  LayoutError that says to solve the model again and save it. Its
  format.json reads {"answer": 0}; 0.2.0 writes
  {"layout": 1, "specsolve": "0.2.0"}. An archive still holds the spec and
  the data it was solved with, under spec.yaml and sources/.

The pull requests since 0.1.0:

• feat(api)!: a saved answer is stamped with layout 1 and the specsolve version that wrote it, and one written by 0.1.0 or earlier is refused by name (#1765)
• docs: each fact about the Python API is stated once, in the docstring of the name it belongs to (#1769)
• docs: the Python API page renders every public name from its docstring (#1766)
• docs(sweeps): the sweep reference says how many sessions a sweep opens on a remote Gurobi (#1771)